Electroetching an aluminum plated semiconductor in a tetraalkylammonium hydroxide electrolyte



NOV 5, 1968 v. c. GARBARINI 3,409,523

ELECTROETCHING' AN ALUMINUM PLATED SEMICONDUCTOR IN A TETRAALKYLAMMONIUM HYDROXIDE ELECTROLYTE Filed March 1o, 196e A L UM/NUM /Nl/ENTOR By V. C. GARBAR/N/ ATTORNEY United Safe Patent 0.

ELECTROETCHING AN I, ALUMINUM PLATED `SEMICONDUCTOR IN A TETRAALKYLAM`` lMONIUM'HYDROXIDE ELECTRGLYTE Victor C. Garbarini, Bethlehem,l Pa., lassignor to Bell Telephone Laboratories, Incorporated, New York, N .Y.,.

a. corporation of New York 1 n .Filed Mar. 10, 196.6, Ser. No. 533,288

. 9\ Claims. (Cl. 204--143) invention relates to" semiconductor devicefabrication land more particularly/'to an electroetching technique for Ina-king metal c'ntacts'on planar semiconductordevices.` .i

The structure and fabrication techniques relating to plaarfsemiconductor devices using ditfusionfand oxide masking are well known In the fabrication of such planar) semiconductor devices it is a standardy practice to deposit the metal films whic'hwill comprise the electrical contacts,"

initallly'over lar-ge areas of the semiconductor material,

usually over an entire slice. Then by further masking randv Accordingly, an obj'ec't of this invention is improvedv planar semiconductor devices.

A more particular object is an improved technique for fabricating planar semiconductor devices.

A specific object is an improved electroetching process for contacts on planar semiconductor devices.

In a specific embodiment in accordance with this invention, a body of silicon semiconductor material is processed to the point where it includes a plurality of diffused junctions with an oxide mask on the diffused surface thereof which defines the area of the metal-to-semiconductor-contacts. Upon this masked surface a thin layer of titanium first is deposited in accordance with the disclosure of Patent 3,106,489, granted Oct. 8, 1963, to M. P. Lepselter. Next, a thicker layer of aluminum is deposited over the titanium and on the aluminum surface a mask is forrned to deline the final configuration of the electrical contacts to the diffused zones. The mask is aligned with and confonms substantially to the underlying contact areas through the oxide mask.

The body of semiconductor material then is immersed in an electrolytic solution with the metal layers connected as the anode. It is important at this point to use an electrolyte which does not have deleterious effects on the semiconductor element, while at the same time producing eiiicient removal of the unmasked metal from the slice surface. In accordance with this invention an electrolyte comprising one from the class of tetraalkylammonium hydroxides provides an excellent rate of etching with sharply defined edges. and with no -apparent deleterious effects. In particular, these electrolytes leave no harmful residue wlhich may ultimately degrade the characteristics of the device.

In this particular embodiment, using a layer of aluminum over a titanium layer the thick aluminum layer is removed by electroetching in a process which is self-limiting inasmuch as the electroetching does not substantially erode the underlying titanium. Accordingly, the titanium remains as an effective electrode until all the unmasked aluminum has been removed. The subsequent step of removing the underlying unmasked titanium is a simple and straightforward chemical etching operation.

3,409,523 d Pateafed Nevp. =,v1ees ice A clearer understanding `of the invention and its otherA objects and features may be had yfrom the following'm'orc detailed description taken' in conjunction Awith the drawing which shows thesuccessiveV s'tepsinthe'fabrication ofV planar semiconductor contacts inaccordance with this invention.

Referring tothe drawing, a` silicon transistor element 10` of the PNP configuration is shown in sectional view. It will be understood that the fabricationpsteps to be describedr generally are accomplished on an entire slice of silicon semiconductor material containingja's many'as'several hundred of such transistor elementsfHowever, for'ease of explanation only a single element, as it wouldfappear afterv separation from ,the slice, is shown. The elementflll is shown at that stage of fabrication at which the oxidemask, 16'deiining the' contact areas is in place and successive'l-a'yzfL ers of metal have been depositedtthereo'ver. In particular,

contacts are applied through the -oxide layer 16 tothe P- type diiiused emitter zone the area 21, and Ilikewise to the N-type diffused base zone in the area 22, which in.

this instance, is ring-shaped. Contact also is made to the collector zone on the top surface of the element in the area 23. The initial layer 17 comprises a-'film of titanium having a thickness of about 200 Angstroms. Olver this layer 17 a second layer 18 of aluminum having a thickness of from 20,000 to 40,000 Angstroms is deposited.

g Next, a photoresist mask 19 is formed in substantial alignment with the Contact areas to the several zones of the element. The techniques for producing such masks using Kodak photoresist'materials areV well known in the art, as exemplified by Patent 3,122,817 to I Andru's.

' In the'next step', as indicated by the' boxll, the slice is immersed in the `electroetching solution comprising, in oneH specific form, a two percent aqueous solution of tetramethylammonium hydroxide at 25 degrees centigrade. Typically, the slice may be suspended in the solution using metal tweezers which comprise one electrode connection. A suitable cathode for the electrolytic process is a small rod of molybdenum, positioned about one inch away from the aluminum surface of the slice. After passing current through the solution for sever-al minutes visual observation indicates completion of removal of the aluminum layer which is not covered by the photoresist mask. A typical etching rate at a current of 200 milliamperes is about 10,000 Angstroms per minute. Thus, for a power transistor having an aluminum layer about 40,000 Angstroms thick, an etching period of four to tive minutes is elfective.

In particular, this electroetching procedure provides relatively sharply deiined edges with substantially no under-cutting of the photoresist mask. Such under-cutting is undesirable for obvious reasons and in particular because of the desirability of having the metal contact electrodes overlap the projections of the PN junction boundaries in accordance with the teachings of M. P. Lepselter in his patent application, Ser. No. 331,168, now U.S. Patent 3,287,612, assigned to the same assignee as this application. The process is useful for all practical thicknesses of aluminum, in particular, from 2000 to about 40,000 Angstroms. It is most advantageous in the 20 to 40 thousand Angstrom range. Moreover, as indicated above, the electrolyte disclosed herein is free of harmful sodium, although of equal effectiveness to sodium Ihydroxide previously used for such processing. This electrolytic process appears advantageous also in comparison with the acid etches which -generally require somewhat longer periods of time during which the KPR mask tends to deteriorate.

The semiconductor element 12 is shown upon the completion of the electroetching step with the aluminum layer coniined to the portions masked by the photoresist layer 19. The next step, as indicated by box 13 is a chemical etching step wherein the slice is immersed in a solution of dilute sulphuric acid and hydrouoric acid'This treat'- ment effectively removes the unmasked underlying thin titanium layer, leaving the sharply deiined contacts as depicted by the semiconductor element' 14. Subsequent fabrication of the device includes removal of the photoresist mask, division of the slice into individual elements, suitable mounting,v application' of external leads and encapsulation of the individual transistor elements.

` Accordingly, there has been disclosedaniadvantagebus electrolytic etching process for the convenient fabrication of aluminum contacts on silicon semiconductor devices. Although the'i'nvention has been disclosed in vt'errnfs of "a specic embodiment, it 'will be' nderstoodftha't alternative arrangements -r'nay be' devised by those s killed'i'nl the 'art which likewise will be within the.y spirit and scopefoftle invention. l .i f'

' More particularly,"althou`gh tetrarriethylamr'nonium hydroxide is a preferred embodiment, 3the'invention*"rkiajl"be practiced 4with any one of the class' lof tetraall'i'yl'lhydrxides denoted by the generic expressionRNL (OH)"",`wh"e're R is any of theseries including ethyl,l methyL'lpropyljbutyl and combinations thereof. The hydroxide s'olutionshould have the characteristics of a strong base to avoid precipitation of dissolved aluminum, lgood electrical conductivity, andleave no harmful residues.

What is claimed is: l

1. 'The method of etching aluminum plated on semiconductor material comprising immer'sng the aluminum plated material in an aqueous solution of tetraalkylammonium hydroxide denoted by the Igeneric expression R4N+(OH), Where R is selected from thexseries consisting of ethyl, methyl, propyfa'rid ybrutyl with a rmetal` cathode member and biasing said cathode negatively with respect to the aluminum plating. p

2. The method in accordance with claim 1 in Which Said semiconductor material is silicon.

3`.' The' method in accordance with claim 2 in Whichsaid solution comprises tetraethyla-mmoniurn hydroxide.

"'4. Tlienethod in accordance with claim 2u in which said solution comprises tetramethylammonimhydroxide. 5. The method in accordance with claim 2 in which lsaid 'solution comprisesteti'apropylammonium hydroxide. v i

6'. The method in accordance lwith claim 2 in which said solution comprises vtetra'b'utylammonium hydroxide.

*'7. The method of forming contact electrodes on diffused jnctionpl'anar silicon transistors comprising defining conf tact areas lonfsilicon surfaces; depositing successive layers 'of vtitariiumarid' aluminum'over said surfaces and Within saididen'ed' `Contact areas', "maskingsaid metal layers, and "iinrne'rsiri'g said-material in 4an aqueous solution'bf tetrame'thylammonium hydroxide *with a metal"ca'thode member 'and bia'sing said cathode negatively with 'respect to the aluminum layer for a lperiod of"tiine' suicient t'o re'r'ntov'e theunrnasked aluminum layer. i

8! Thel'metlrodin accordance with claim'3 in which said titanium'la'yer has `a thickness of 200 An'gstrorrisy and said aluminum layer-has `a thickness'of about 2000 to 40,000 ngstrms. f "i 'f 9. Awmethod inaccorda'nce with'claim 3 in which said aluminum' layer has a thickness of about 20,000 to 40,000 Angstro'ms and said bias is eifective to produce a'current 'of about 200 milliamperes for -a' period of from three to fW minutes. 'v

` References CitedA UNITED STATES PATENTS RBERT KrMIHALEK, Primary Examiner. 

1. THE METHOD OF ETCHING ALUMINUM PLATED ON SEMICONDUCTOR MATERIAL COMPRISING IMMERSING THE ALUMINUM PLATED MATERIAL IN AN AQUEOUS SOLUTION OF TETRAALKYLAMMONIUM HYDROXIDE DENOTED GENERIC EXPRESSION R4N+(OH)-, WHERE R IS SELECTED FROM THE SERIES CONSISTING OF ETHYL, METHYL, PROPYL AND BUTYL WITH A METAL CATHODE MEMBER AND BIASING SAID CATHODE NEGATIVELY WITH RESPECT TO THE ALUMINUM PLATING. 